This invention relates to material testers and more particularly to a novel and improved system for directing a search unit of an ultrasonic nondestructive material tester, for example, over curved or sloped surfaces or the like.
Presently it is conventional to inspect a workpiece for flaws or defects contained therein by ultrasonic nondestructive testing methods. These methods include an ultrasonic transducer such as a piezoelectric crystal which when energized by an electrical stimulous radiates a beam of ultrasonic energy into a workpiece. If the familiar pulse echo search unit is used, the ultrasonic energy is reflected back from a defect within the workpiece to the crystal whereby the mechanical vibrations thereof are translated back into electrical signals. In this method the time of arrival of the return signal indicates the presence and the exact location of the defects within the workpiece. On the other hand, when the familiar "through" transmission system is used a separate transducer is aligned opposite the workpiece and attenuations due to the defect in the workpiece or change in the time of arrival of the signal is indicative of some characteristics of the workpiece.
It has been found in ultrasonic systems that the pulse/echo system is generally suited for large workpieces whereby the search unit must scan the workpiece to make sure that the entire area of the workpiece is inspected for defects. Normally alternate scans and indexes are made over the workpiece surface. On the other hand, if conical, frusto-conical, hemispheric or other such shaped workpieces are used, they are affixed to a rotating table and an indexing search unit is used whereby the index motion occurs after each complete revolution of the turntable.
As is well known to those skilled in the art, ultrasonic energy is greatly attenuated in air; thus it is oftentimes necessary to immerse the workpiece and the search unit into a couplant material, such as water, so that good ultrasonic coupling is possible. Thus it is necessary on large workpieces, for example, to have a large test tank filled with water or other suitable couplant. The search unit is then scanned relative to the workpiece while both are immersed in the couplant fluid in the test tank.
In normal operation it is found that when inspecting flat workpieces a search unit only needs to be operated along the horizontal (X and Y) axes of the workpiece. But difficulty exists when the workpiece is of an irregular shape. For example, the workpiece may have sloped or curved surfaces. In order to compensate for the irregular shape such as the slopes and curves hereinafter mentioned, a vertical (Z) axis and sometimes a gimbal (.theta.) axis of movement is necessary for the search unit. Not only must the search unit move in the X and Y axes but also sometimes in the vertical (Z) and gimbal (.theta.) axis. As an example, a scan is only made in a single direction (X) but indexing may be necessary in two other axes (Y and Z) to advance the search unit parallel to the curved or sloped surface while gimballing (.theta.) may be simultaneously necessary to correct the search unit attitude with respect to the part surface.
Heretofore such devices capable of causing the three axis motion has included very complicated electrical tape program reading systems and digital and/or analog computers for deriving the voltage to change the positions of the index axes (Y and Z) and gimbal (.theta.) axis. As can be seen, such tape control devices in computers for performing such functions are quite expensive. Thus, in most cases, programmed indexing and gimballing has been performed manually. While of course this later operation is less expensive, it is undesirable to provide an operator for continuously indexing or moving the position of the search unit for each scan of the search unit.
An example of the type of workpieces which are difficult to scan during ultrasonic testing are sloped workpieces. On these workpieces, a simultaneous position change between the Y and Z axes is necessary during the indexing so that the angle of the hypotenuse between adjacent sets of Y and Z axes coordinates remains fixed. Another type of workpiece, which is difficult to inspect, is a curved workpiece such as a convex or concave surface, for example, a hemispherically shaped dome, whereby the angle of the hypotenuse continually changes thereby describing a curved indexing path.
It is, therefore, extremely desirous of providing an electrical control system which can control the axis position of a three dimensional object so as to perform the defect search maneuvers on these workpieces which have heretofore been difficult to scan.
A system which performs similar functions and illustrates some of the mechanical mechanisms used in conjuction with the present invention is shown in U.S. Pat. No. 3,721,118 assigned to Automation Industries, Inc., the assignee of this invention. The similarities in these mechanica portions of this invention are adequately described in the prior patent and will not be described in detail herein.